An essential element of modern mobile communications systems is the “cell site.” The cell site includes one or more cellular base station antennas aimed at a desired geographical area of coverage with coaxial cables connecting the antennas to base station radio equipment. The performance of a cell site is often limited by passive intermodulation (“PIM”) interference. PIM interference occurs when the high-power downlink signals (the “main beam”) transmitted by the base station antenna mixes at passive, non-linear junctions in the RF path, creating new signals. When these new signals (intermodulation products) fall in an antenna's uplink band, they act as interference and reduce the signal-to-interference-plus-noise ratio (“SINR”). As the SINR reduces, the geographic coverage of the cell site reduces and the data capacity of that cell site reduces.
It is well documented that loosely touching metal-to-metal surfaces can behave in a non-linear fashion and become sources of passive intermodulation when illuminated by high power RF signals. Recently, it has been determined that loose metal-to-metal connections located behind base station antennas are also able to generate high levels of passive intermodulation. Even though this region is well outside the main beam of the antenna, enough RF energy is present in this region to excite non-linear objects and generate PIM. Metal brackets and associated hardware for supporting RF, optical, ground and remote electrical tilt (“RET”) cable are common sources of loose metal-to-metal contact found in the region behind and close to base station antennas.
A common method for mechanically supporting base station cables utilizes two plastic clamp blocks that fit around one or more cables. An example of this style cable support block is disclosed in Jobin et al, U.S. Pat. No. 5,794,897. In this type of conventional cable support, a ⅜-inch or 10-millimeter diameter stainless steel threaded rod is inserted through the support block halves and stainless-steel hardware is installed to clamp the plastic block halves together on the threaded rod.
In another conventional configuration, a steel strut is used to secure multiple threaded rods for supporting plastic cable support blocks and other components. One end of the threaded rod engages with (screws into) a spring-loaded strut retainer captured inside the strut. An example of this style of spring-loaded strut nut is shown in FIG. 4 of Rebentisch, U.S. Pat. No. 4,784,552, which is reproduced in FIG. 1 (prior art) of this disclosure with the shading and the original element numerals removed, and the new element numerals referenced in the following discussion added. Referring to FIG. 1, the spring-loaded strut nut 10 can be removably tightened to different places along the elongated steel strut 11, which extends in a longitudinal direction (into the page) transverse to the cross-section shown. A threaded rod 12 passes through a hole in a lock plate 13, which is positioned across a passage 14 between two strut rails 15a-15b that leads into a channel 16 of the strut 11. The threaded rod 12 engages with (screws into) a central hole in a captured nut 17 located in the channel 16. The width of the captured nut 17 is slightly less than the width of the channel 16, which allows the captured nut to slide along the strut in the longitudinal direction while the strut prevents the captured nut from rotating within the channel. A bolt head or compression nut 18 engaged with the threaded rod 12 can be tightened to secure the strut rails 15a-15b between the lock plate 13 and the captured nut 17 to lock the strut nut 10 in place at a desired position along the strut. While the threaded rod 12 shown in FIG. 1 terminates in a bolt head in this particular example, the threaded rod may alternatively pass through a compression nut and extend further away from the strut, which allows a cable support block or another component to be attached to the threaded rod 12.
The strut nut 10 also includes a spring 19 attached to the bottom of the captured nut 17 and located in the channel 16 between the captured nut and the bottom side of the strut, which biases the captured nut 17 toward the strut rails 15a-15b . The spring 19 helps to stabilize the captured nut 17 while it slides along the strut 11 and receives the threaded rod 12 used to tightened the strut nut 10 at a desired position along the strut. This mounting system allows the strut nut 10 to easily slide along the longitudinal axis of the strut 11 into a desired location before being secured in place by tightening the bolt head or compression nut 18 on the threaded rod 12 to lock the strut nut in place. A number of these strut nuts 10 may be positioned in this manner at multiple location along the longitudinal axis of the strut 11 to position multiple cable support blocks and other components in desired positions along the strut.
For example, as shown in FIG. 2 (prior art), a compression nut 20 may be engaged onto the threaded rod 12 and tightened against the lock plate 13 of the strut nut 10 to secure the strut nut in a desired position along the strut 11 with the threaded rod extending beyond the compression nut further away from the strut. This allows an additional component to be attached to the portion of the threaded rod 12 that extends beyond the compression nut 20. In the particular example shown in FIG. 2, the threaded rod 12 extends through a cable support block 21, which supports several coaxial cables 22 at a desired support location. In this example, an end nut 22 is engaged onto the threaded rod 12 and tightened to secure the cable support block 21 between the compression nut 20 and an end nut 22. In a conventional configuration shown in FIG. 3 (prior art), the strut 11 is secured to an antenna mounting pipe 30 using a pair of saddle brackets 31, 32. The strut 11 can be attached to other types of support structures, such as walls, floors, towers, cabinets, angle supports, and so forth.
Multiple sources of passive intermodulation are present with the conventional cable support system described above. First, the strut 11, the threaded rod 12, and the lock plate 13 are typically made of steel with a galvanized or electroplated zinc finish, while the compression nut 20 and the end nut 22 are typically produced from stainless steel. Stainless steel and galvanized steel are at opposite ends of the galvanic series. Over time, corrosion will occur at the dissimilar metal interface creating a source of PIM.
Second, the saddle brackets 31, 32 used to secure the strut 11 to the antenna mounting pipe 30 typically has large surface areas of metal-to-metal contact between the strut and the saddle bracket. It is difficult to maintain a high clamping force over the full contact area between the strut 11 and the saddle brackets 31, 32 resulting in inconsistent metal-to-metal contact, which can also generate PIM.
Third, the strut 11 is typically a steel component purchased in 8-foot or 10-foot standard lengths coated with a galvanized finish for corrosion protection. The standard lengths are often cut in the field to the desired lengths, which exposes the cut surfaces to the weather without corrosion protection. This allows rust to form on the cut faces, which can become another source of PIM.
An improved cable support system is therefore needed to overcome these drawbacks experienced by conventional cable support systems.